Although some of the methods of joining aluminum bodies of automobiles is spot welding, riveting, etc., there is several problems such that it requires complicate piping structures on a robot (for example, supply and ground of electricity, supply and drain of water, and supply and drain of oil), and causes an undesirable working environment with large noises and electrical sparks, etc.
To solve such a problem, there is proposed a spot welding apparatus for example, in German Patent Application Laid-Open DE 197 31 638, International Publications WO 98/04381, WO 01/28732, and WO 01/036144, which carries out a friction stir welding of two over-wrapped work pieces by pressing a high-speed rotating tool against one outer surface of the work pieces.
FIG. 7 is a view showing a configuration of a spot welding gun 1, which is a conventional friction stir welding apparatus, utilizing the technologies of the above disclosures. This spot welding gun 1 comprises a rotating tool 3 having a pin 2 at a tip end thereof, and presses the rotating tool 3 against one of work pieces (not shown) while rotating the rotating tool 3 at high speed, and then softens and stirs a joining part of the work pieces with the pin 2 with the frictional heat by rotation, and thus joins the work pieces together.
The spot welding gun 1 also comprises a rotational movement drive source 4 for rotating the rotating tool 3 generally oriented downward about an axis-of-rotation L thereof at a high-speed, and a linear movement drive source 5 for linearly moving the rotating tool 3 along the axis-of-rotation L. The rotating tool 3 is fixed to the lower end of a rotational shaft 7 being rotated about the axis-of-rotation L by the rotational movement drive source 4. This rotating tool 3 is limited its linear movement along the axis-of-rotation L, and is held by a header 10 so as to be rotatable about the axis-of-rotation L. Moreover, a pressurizing shaft 6 is provided so as to surround the rotational shaft 7 and the header 10 is fixed to the lower end of this pressurizing shaft 6.
An outer peripheral of the pressurizing shaft 6 is formed to be an outer threaded portion onto which a nut member 11 is threadedly engaged. These are implemented by for example, a ball-screw mechanism. As the nut member 11 is rotated by the linear movement drive source 5, the pressurizing shaft 6 moves upward or downward along the axis-of-rotation L. Since the rotating tool 3 is rotatably supported through the header 10 at the lower end of the pressurizing shaft 6, the rotation of the rotational shaft 7 by the rotational movement drive source 4 causes a high-speed rotation of the rotating tool 3 as well as the linear movement.
The above mechanism is attached to the main frame 14. To the main frame 14, a gun arm 12 is fixed extending downward from to the main frame 14. The lower portion of the gun arm 12 is bent and a receiving portion 13 which opposes the rotating tool 3 is provided at a tip end of the lower portion. With this configuration, by moving the rotating tool 3 downward while rotating it at a time of welding, work pieces are pinched between the receiving portion 13 and the rotating tool 3 to be applied a pressure force and therefore spot welding is carried out.
However, the pressurizing shaft 6 which surrounds the rotational shaft 7 is supported by the main frame 14 through the nut member 11. When the header 10 is pulled up to the uppermost position, since an immediate upper part of the header 10 is supported by the main frame 14, the shaft is stably supported. However, when the pressurizing shaft 6 is lowered, it will be difficult to stably support the rotating tool 3 with high-speed rotation since a distance between the supporting position of the pressurizing shaft 6 and the headers 10 becomes large. Meanwhile, at the time of welding, the work pieces will be pressurized while the distance of the supporting position and the rotating tool 3 becomes large. However, since a pressure force is only transmitted through the hollow pressurizing shaft 6, it is difficult to stably transmit the pressure force with the thin pressurizing shaft.
Therefore, in order to ensure stably lowering of the rotating shaft at high rotational speed and positively applying a pressure, it is only necessary to choose a larger diameter for the pressurizing shaft 6. However, using of the larger diameter for the pressurizing shaft 6 will result in a larger ball screw mechanism, pulley, etc., and thus a new problem will arise that the size of the whole apparatus becomes larger.